1. Field of the Invention
The present invention relates to improved sol-gel barrier films. More particularly, the invention pertains to sol-gel coated polymer films which do not require thermal or electron beam densification to obtain useful oxygen barrier films.
2. Description of the Prior Art
It is known in the art that SiOx barrier coatings may be produced on substrates either by thermal evaporation of silica dioxide or plasma enhanced chemical vapor deposition of organo silicones. Both of these processes are expensive due to materials costs or processing costs. However they are the only currently available methods available to glass coat polymer films because these processes do not destroy the integrity of the films. U.S. Pat. No. 5,084,356 illustrates vapor depositing SiO.sub.2 onto a substrate. An alternate method of producing silica films is by the sol gel method. This process requires densifying the sol gel coating to SiO2 by increasing the Si-O-Si bond content and eliminating water in the process. Two methods are available to accomplish this. One requires that the monomeric sol-gel coating be heated to over 800.degree. C. and the other requires the sol-gel coating to be exposed to an electron beam. U.S. Pat. Nos. 5,318,857; 5,186,745; 5,091,009; 5,013,588; 4,997,482; 4,385,086; 4,361,598; 5,096,738; 5,091,224; 4,614,673 illustrate the coating of a monomer followed by in-situ cure by heating. These methods have disadvantages that effect the choice of substrate and the economics of the coating process.
The process of the present invention produces a glass-like coated polymer film by the sol-gel method without thermal or electron beam densification to obtain a useful oxygen barrier film. According to the process, a relaxed sol-gel composition is formed by hydrolyzing a tetrafunctional alkoxide silicate in an aqueous solution including a Lewis acid or metal chelate catalyst to produce a pre-crosslinked polymer having a viscosity of about 2600-3200 cps. The crosslinked polymer is then relaxed without depolymerizing it by diluting to a viscosity of about 1 cps. This is then coated on a substrate and dried without undergoing a curing. None of the aforementioned patents show coating of a polymer. Rather they coat monomers and then cure in-situ.
U.S. Pat. No. 4,842,901 precondenses tetraethyl ortho silicate, water, acid and a C.sub.4 or higher alcohol, deposition on a substrate and drying, however there is no redilution to relax the polymer. U.S. Pat. No. 4,966,812 shows a reliquification by an ultrasonic breaking of polymer bonds with subsequent dilution, however, there is no dilution of a fully condensed polymer composition and this disclosure does not show use of a Lewis acid. U.S. Pat. No. 4,842,837 discloses hydrolyzing alkoxysilane with an alkaline catalyst and U.S. Pat. Nos. 5,328,645; 4,741,778; and 4,789,563 show the use of an alkaline hydrolysis or condensation. However, alkaline catalysts form polymer particles which produce a non-uniform coating. None of the foregoing patents disclose preformation of a cured polymer using a Lewis acid, relaxing the polymer by redilution and coating a cured polymer with subsequent drying. This invention produces a high oxygen barrier film on the substrate that is stable to heat and moisture while being extremely thin film (preferably &lt;0.1 micron) and thus eases the recycling of the material.